Furnace



A. N. OTIS Nov. 10, 1936.

FURNACE Filed Nov. 23, 1934 Inventor: Albert N. Otis,

His Attorney.

and-

Patented Nov. 10, 1936 UNITED STATES FURNACE Albert N. Otis, Schenectady, N. Y., assignor to General Electric Company. a corporation of New York Application November 2:, 1934, Serial No. 754,474 6 Claims. (01. 286-3) My invention relates to furnaces for continuously heat treating metal strip. In certain forms of such furnaces, as heretofore constructed, the strip is passed through the furnace horizontally 5 while supported by a series of rollers. Where the temperature to which the strip is raised is relatively high, such for example as is necessary to normalize steel strip, it has been found that with such a furnace arrangement the surface of the strip was often scratched or pitted by the rollers and that the value of the strip was impaired for use where a material with a smooth surface is required. To avoid this trouble the strip may be heated and cooled while it hangs is unsupported vertically in a long loop but such an arrangement while useful for treating strip which isthin and easily flexed cannot be employed where relatively thick strip is to be treated without involving a furnace construction and 2 equipment of such proportions as to be impracticable.

It is one object of my invention to provide an improved furnace for continuously treating strip of all usual commercial thicknesses which will avoid any scratching or pitting of the strip being treated. Another object is the provision of a furnace of this type with which strip may be heat treated at a relatively high speed and which is simple in construction and economical in operation.

In the furnace which I have devised and am about to describe the strip to be heat treated is supported as a flat catenary, the heating and subsequent partial cooling of the strip being effected while it passes through the catenary untouched by any supporting means which might scratch or pit the surface thereof.

In the more complete form of my invention which is adapted for a large installation or where a greater speed of the strip is desired, I have provided means for preheating the strip, the furnace being in efiect a combined roller hearth and catenary furnace. The strip passes first through the heating chamber of the roller hearth furnace where its temperature is raised to a point preferably just below that at which its surface might be affected by the supporting means therein. It then passes on into the heating chamber of the catenary furnace where its temperature is further raised to the point necessary for the desired heat treatment thereof and while still in the catenary, untouched by any supporting member it passes on into the cooling chamber. The latter chamber is provided with means to effect 65 the rapid cooling of the hot strip to give the strip ing through the furnace.

the desired grain structure and working characteristics.

The degree of cooling of the strip while in the catenary needs be only suilicient for the desired heat treatment requirements and for avoiding any possible injury to the surface of the strip by the supporting means at the latter end of the catenary. To further cool the strip before exposing it to the outside air I have provided a second cooling chamber continuous with the first to which 10 the strip passes and in whichit may be supported in any suitable manner since there is no danger at that time of injuring the surface thereof since the strip is then relatively hard.

By the apparatus thus briefly described strip 15 of any ordinary commercial thickness may be heat treated for, during the entire time that the temperature .of the strip is above that point at which it is susceptible to surface injury by any supporting means therefor whether the strip is n being heated or cooled, it hangs freely in a catenary and since the furnace is horizontally disposed and the catenary is fiat the strip needs to undergo but a small amount of bending in pass- Moreover by the proa vision of the preheating chamber, namely the roller hearth furnace, a more economical construction results since the strip may be given a considerable portion of its final heat before it passes into the catenary thus permitting a mini- 30 mum length of the latter and a higher speed of the strip through the furnace.

My invention will be better understood from i the following description taken in connection with the accompanying drawing, and its scope 35 will be pointed out in the appended claims.

Referring to the drawing, 'Fig. 1 is a longitudinal section of a furnace construction illustrating one embodiment of my invention; Fig. 2 is a cross sectional view taken on line 2-4 of Fig. 1; 40 Fig. 3 is a view similar to Fig. 1 showing a modification;.and Fig. 4 is a circuit diagram of the electrical apparatus for controlling the movement of the strip through the furnace.

In Fig. 1 of the drawing the strip which is to 5 receive the heat treatment is represented at I. This strip may be withdrawn from a coil and fed to the furnace by means of the pinch rolls 2 and is finally withdrawn from the furnace by the pinch rolls 3, the two sets of rolls being driven 50 by suitable driving means, to be described later, whereby the strip is driven at the same speed by each set. The stripfirst enters the preheating chamber 4. This chamber as shown comprises the outer metal casing 5 supported in elevated position by the structural steel framework i. Within the casing 4 is the layer I of heat insulating material and an interior lining I of suitable heat refractory material. Adjacent the upper and lower walls of this chamber are the electric heating elements 4 whereby the chame her is maintained at the desired temperature which, for example, may be about 1100" 1".

Within the chamber 4 the strip is supported by suitable means such as the rolls ill one of which is arranged adjacent the outlet opening of the chamber through which the strip passes into the heating chamber 12 of the catenary furnace where the temperature of the strip is further raised to the point necessary for the desired heat treatment thereof.

The chamber i2 is somewhat deeper than chamber 4 but its construction is similar thereto since it also comprises the outer metal casing II, the intermediate layer l4 of heat insulation and the lining ll of refractory material. This chamber like chamber 4 is provided with the electric heating units It arranged on the top and bottom walls thereof by which the chamber is maintained at the desired temperature which, for example, may be about 1800'. Connecting with the chamber I2 is the cooling chamber is which as shown inclines upwardly from the chamber l2 so-as to approximately follow the contour of the strip as it hands in a catenary. This chamber It is represented as comprising the metal casing 20 which at its forward end is secured to casing l3 and is supported by the structural steel supports 2|. At the terminal end of the cooling chamber is the strip is supported by suitable means such as a roll or rolls shown at 22 between which rolls and the last roll II in the preheating chamber the strip is adapted to hang free from contact with any other body which might scratch or injure its surface while it is in such a heated condition as to be susceptible to such injury.

Adjoining the cooling chamber is is the second cooling chamber 24, through which the strip passes and in which the temperature of the strip is further reduced to the point where the strip may safely be passed into the outside air without danger of oxidation. The chamber 24 is iilustrated as having a construction similar to that of chamber I! in that it comprises the sheet metal casing 28 supported in an elevated position by the structural steel supports 28. The chamber is provided with suitable means for supporting the strip therein, such as the rolls I1, and has a suitable gas seal 14 at the terminus thereof which in conjunction with the seal 2! at the entrance end of the preheating chamber 4 prevents the escape of the protective atmosphere which ordinarily would be used in the furnace.

Cooling of the strip in chambers II and 24 at the desired rate may be eifected merely by the radiation of heat from the sheet metal casings forming the chambers. If a greater rate of moling is desired any one of various well known expedients may be employed; for example, the casings may be provided with water jackets such as that shown at If and 33 or may be provided with cooling pipes arranged within or without the casing in any common manner. Moreover the gas forming the protective atmosphere may, if desired, be caused to circulate through the casings and a cooler in the manner disclosed in the Bteenstrup Patent No. 1,531,039 and if regulation of the rate of cooling of the strip by the gas is desired to give the strip particular characteristics it may be effected in the manner disclosed by my copending application, Serial No. 742,132, filed August 30, 1934.

In certain cases I may construct tbepreheating chamber 4 with a slight upward inclination in the direction of movement of the strip and likewise construct the second cooling chamber with a slight downward inclination so as to bring the seals 2! and II more nearly at the same level as the lower portion of the main heating chamber. By so doing any objectionable chimney effect due to the difference in density of the gas in the furnace and the end chambers would be avoided.

Although the furnace is completely closed, it may be found desirable to suppLv it constantly with a small quantity of the protective gas to make up for leaks that may occur therein particularly at the seals 24 and is in order that no air shall gain entrance into the furnace during its operation. For this purpose I provide the furnace with the pipe 40 by which a suitable amount of the protective gas can be supplied constantly thereto. In the above construction the strip is driven solely by the two sets of pinch rolls 2 and 25 4, the strip being supported within the furnace by means of the rolls ll, 22, and 21 which rotate freely and which, as shown in Fig. 2, are mounted in bearings 42 which are enclosed in the outward extensions 43 of the chamber casing. In certain cases it may be found desirable, however, to drive one or more of the rolls ll, 22, 21.

In the modified form of my invention shown by Fig. 3 the strip first enters the roll enclosing chamber 44 from which it passes on into the catsnary heating chamber. In this case it will be obvious that the temperature in the catenary chamber l2 and the speed of the strip must be so related that the strip is able to acquire the desired temperature therein and before it enters the cooling chamber II. with this form of furnace one may not be able to obtain the economies in production as may be obtained with that form of my invention shown by Fig. i inasmuch as the speed of the strip will be inherently less than in the case of the form shown in Fig. 1 where the strip has its temperature raised in the preheating chamber to a point just below that at which it would be scratched or become pitted by the last roll in that chamber. The modified form shown by Fig. 3 obviously may be provided with the same cooling means as shown in Fig. l but which has purposely been omitted in the interest of greater simplicity in the drawing.

Inasmuch as the strip i hangs in a relatively flat catenary in the furnace, and since the strip in the catenary for reasons already pointed out above must not be allowed to touch any part of the furnace, I have provided means for driving the two sets of pinch rolls 2 and 3 at the same peripheral speed which will maintain the catenary in a substantially fixed position. Such means comprises motors 4i and 41 connected with worm gearing to the respective pinch rolls 2 and 8. These motors preferably operate on direct current which in the present case is shown as dcrived from the direct current generator 4| which is shown direct connected to the three phase alternating current motor 4! connected to the three phase source of supply ll. The field circuit of generator 48 is supplied from the direct current source II and is controlled by the rheostat 42 whereby the generator voltage may be adjusted and the two motors driving the pinch rolls may be operated at any desired speed.

It will be understood-by'those skilled in the" art that as a practical manner the two motors 46 and 41 of themselves cannot be depended upon to operate continuously to drive the strip at exactly the same speed at the two ends of the furnace; Slight variations in construction of the two motors and of the pinch rolls, differences in temperature of the strip engaged by the two sets of pinch mils and particularly the elongation of the strip due to its higher temperature at the point of leaving the furnace than at the entrance point necessitate some special means for insuring that the two motors operate at such relative speeds as to maintain the catenary in the proper freely hanging position in the furnace. For this purpose I have provided the means which I shall now describe for automatically regulating the speed of the motor driving pinch rolls 3. I i

The cooling chamber i9 and the heating chamber I! are provided with openings through which light beams are arranged to be projected from suitable light sources represented at 55. These beams are received by suitable photoelectric tubes 56. The beams are arranged one vabove the other and are so related to the catenary that when the latter has the proper depth the upper light beam is intercepted by it while the lower beam passes freely beneath it. Under these conditions the field winding 51 of motor 41 is energized from the. source of supply i through the resistor 58, the relay 5!! short circuiting resistor 60, and through the rheostat 61. It is to be noted that the field winding 62 of the other motor 46 is supplied directly from the source v5i whereby the field of that motor remains constant or any variation in voltage of source 5| will effect both motors equally. 1

.Should the speed of the motor 41 become too great so that the bottom of the catenary rises sumciently to allow the upper light beam to engage the upper photo tube, this tube through the amplifier 64 will energize the winding of the relay, 65 thereby closing the upper contacts of that relay and energizing relay 66. This relay then closes a short circuit about the resistor 58 and the resulting increased voltage applied to the field winding 51 causes a material reduction in the speed of motor 41. at which the pinch rolls 3 now pull the strip causes the depth of the catenary quickly to increase thereby restoring the catenary to its original' depth at which it will again intercept the upper light beam. At such interception relays 65 and 66 each will be restored to its original condition. Should the speed of the motor become too low so that the catenary would intercept the lower light beam, the lower photo tube 56 through the amplifier 61 cooperating therewith would cause the energization of relay 69. In operating, this relay will close its upper contacts thus closing the circuit of the relay 59 by the operation of which the shunt around the resistor 60 will be opened. The resulting decrease in voltage applied to the field winding 51 will cause the motor 41 materially to increase in speed thereby quickly raising the catenary to its original position where the lower light beam is no longer intercepted. Upon the restoration of the lower light beam relays 69 and 59 will automatically become restored to their original condition. By the above described apparatus a temporary correction is quickly made to' the speed of motor 41 in accordance with the depth of the catenary.

The decreased speed stat produces but small changes in the resistance of the field circuit of motor 41. Rheostat 6| is shown operated through a worm gearing driven from the reversible motor 12 which is provided with the reversely wound series field windings 13 and 14. These windings connect with the supply circuit 5| through the upper contacts of relays 65 and 69 while the armature of motor 12 connects with the supply circuit through the contacts of the relay 15. Relay 15 is normally energized being connected with the supply circuit 5! through the bottom contacts of both relays 69 and 65 in series. Relay 15 is provided with a time delay opening device 16 whereby when the relay winding is deenergized by the operation of either relay 65 or 69 the motor circuit controlled by this relay remains closed for a predetermined length of time, which, for example, may be a few seconds only. During the brief time interval that motor 12 operates it effects a slight change in the setting of the rheostat 6| whereby motor 41 has its, speed either increased or decreased a slight amount and in a direction which will tend to correct its speed and maintain the catenary with the desired depth. It will be noted that each time the depth of the catenary is restored to its original condition both relays 65 and 69 become deenergized whereby the circuit of motor 12 is held open and the circuit of relay 15 is held closed until the next time that a change in speed of motor 41 is made necessary. It will also be noted that the corrections causedby operation of relay 15 are definite, depending on the time setting of the delay device, and are in no way affected by the length of time that relay 65 or 69 may remain closed.

I have chosen the particular embodiments described above as illustrative of my invention and it will be apparent that various other modifications may be made without departing from the spirit and scope of my invention which modifications I aim to cover by the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A horizontally disposed furnace for continuously heat treating heavy steel strip comprising a heating chamber having an inlet opening at one end and near the top thereof and an outlet opening at the opposite end and near the bottom thereof, a cooling chamber connecting with said outlet opening and idler rolls located external to the heating chamber and arranged to support the strip freely therebetween as a catenary in said chambers, there being approximately equal portions of the catenary in each of said chambers whereby after being heated the strip is cooled at a rate to produce the desired change therein and to a point at which it is not damaged by contact with the following supporting idler roll.

2. A horizontally disposed furnace for continuously heat treating metal strip without injury to the surface thereof comprising a heating chamber, a cooling chamber connected at one end therewith. means for moving a continuous strip progressively through said chambers, and rolls for supporting the strip freely as a flat catenary in said chambers. one of said rolls being arranged outside of said heating chamber and the other roll being arranged in said cooling chamber and adjacent the opposite end thereof where the strip becomes cooled to a temperature at which the surface thereof is not injured by said other roll.

3. A horizontally disposed furnace for continuously heat treating metal strip without injury to the surface thereof comprising a preheating chamber. a heating chamber connected therewith. a cooling chamber connected at one end with the heating chamber. means for supporting said strip freely as a hat catenary in said heating and cooling chambers comprising a roll in said preheating chamber and a roll in said cooling chamber adjacent the opposite end thereof whereby the temperature of the strip at both rolls is below the point at which the rolls may injure the surface of the strip.

4. A horizontally disposed furnace for continuously heat treating heavy strip without injury to the surface thereof comprising means for freely supporting a portion of the strip as a catenary, means for continuously moving the strip through the catenary, means for heating the strip in one part of the catenary to effect said treatment and means for cooling the strip in the following part of the catenary to a temperature below that at which the surface thereof would be injured by said supporting means.

that at which the strip would be injured by the supporting means therefor.

6. A horizontally disposed furnace for continuously heat treating burnished metal strip without injury to the surface thereof comprising a heating chamber having heating elements therein. a cooling chamber connected therewith at one end thereof, a preheating chamber having heating elements therein and connected with the heating chamber by a narrow opening, rolls for freely supporting said strip as a catenary in said chambers and said opening, one of said rolls being arranged in the preheating chamber adjacent said opening and the other roll being arranged adjacent the distant end of said cooling chamber whereby the rolls are protected from the heat of the heating chamber and the strip while passing through the catenary is heated and subsequently cooled to a point at which its burnished surface is not injured.

ALBERT N. OTIS. 

